Batteries are commonly used to power many types of motors and electronic devices for use in portable applications. The battery may be rechargeable or disposable (one-shot usage) type. The battery provides operating power for integrated circuits in portable electronic systems, or provides an electromotive force to drive motors for industrial applications.
One area of particular interest is automotive power trains. Battery-powered automobiles offer many interesting possibilities for fulfilling transportation needs, while reducing energy consumption, and minimizing hazard to the environment. Automobile batteries must be rechargeable and preferably deliver high voltages, e.g. greater than 4.5 VDC, to provide adequate power to the motor. The battery should also have good electrochemical stability, safety and longevity.
Common types of rechargeable battery are known as lithium-ion cell and lithium metal cell. U.S. Pat. Nos. 5,460,905; 5,462,566; 5,582,623; and 5,587,253 describe the basic elements and performance requirements of lithium batteries and their components. A key issue in the development of high energy batteries is the choice of the electrolyte element to improve the possible output voltage, stability, cycle life, and safety of the battery. A large number of non-aqueous organic solvents have been suggested and investigated as electrolytes in connection with various types of cells containing lithium electrodes. U.S. Pat. Nos. 3,185,590; 3,578,500; 3,778,310; 3,877,983; 4,163,829; 4,118,550; 4,252,876; 4,499,161; 4,740,436; and 5,079,109 describe many possible electrolyte element combinations and electrolyte solvents, such as borates, substituted and unsubstituted ethers, cyclic ethers, polyethers, esters, sulfones, alkylene carbonates, organic sulfites, organic sulfates, organic nitrites and organic nitro compounds.
One class of organic electrolyte solvents that have received attention as a component of electrolyte elements for electrochemical cells and other devices are the sulfones. Sulfones can generally be divided into two types: the aromatic sulfones and the aliphatic sulfones. The aliphatic sulfones can also be divided into two types-the cyclic (commonly referred to as sulfolanes) and non-cyclic. The non-cyclic aliphatic sulfones form a potentially-attractive group of organic solvents that present a high chemical and thermal stability.
In particular, ethyl methyl sulfone (EMS) has shown remarkable electrochemical stability, reaching 5.8v vs. Li/Li+ by a conservative stability criterion. For example, 2M LiPF6/EMS has been used as the supporting electrolyte in a dual graphite cell which operates around 5.5v, and 1M LiPF6/EMS has been used as the electrolyte in Li/T2-Li2/3[Ni1/3Mn2/3]O2 cell which operates up to 5.4v. Despite its success as solvent in those cases, EMS applications are limited by a shortcoming, i.e. its relatively high melting point, 36.5° C., which eliminates its use as a single solvent in electronic devices whose range of operation includes temperatures much below ambient. To overcome this limitation EMS must be blended with other high stability solvents that yield low-melting eutectics with EMS, or be replaced by alternative sulfones with lower melting points.
A eutectic mixture of EMS with dimethyl sulfone melts at 25° C., and the mixture has been used in lithium salt solution conductivity studies extending well below ambient, however, crystallization takes place on long exposure to low temperatures. Since single solvent electrolytes are desirable for a variety of reasons, and since alternative second components to provide lower-melting eutectics than the above are also desirable, a need exists to further study the synthesis of sulfone-containing molecular liquids.